Dry mixing of a small amount of one kind of particles into a larger amount of a different kind of particles is used, e.g., for mixing of enzyme particles into powder detergents or animal feed.
The current technology used to mix enzyme granules into powdered detergent products includes rotary drum mixers, which are able to mix small amounts of granular enzyme products efficiently without causing damage to the enzyme granules. Drum mixers are, however, large and costly process equipment. In addition is takes up substantial amount of space in the detergent manufacturing plants. This makes the introduction of enzymes into new plants costly and difficult. Mixing of liquids with liquids or mixing of gases are on the contrary done very easily and efficiently using so-called static mixers, which are very compact and inexpensive equipment and used for many years throughout the process industry. Application of static mixers for mixing powders are, however, very scarce. Only few papers have been published on attempt to use static mixers for mixing dry or essentially dry powder streams. Powders are inherently more difficult to handle than liquids. The multi-phase nature of powder flows are still not very well understood despite the advancement of sophisticated modeling and simulation tools becoming available; such as CFD (Computational Fluid Dynamics) and DEM (Discrete Element Method). No single theory is, however, able to accurately describe powder flows. These difficulties have led the industry to staying with current mixing technology such as rotating drum and paddle mixers.
Generally the same technology is used to produce blends of two or more kinds of enzyme particles, e.g. enzyme granules with different kinds of detergent enzymes.
Static mixers have the drawbacks of being difficult to apply for powder mixing due to the lack of understanding of powder flows and mixing. In addition the mechanical constructions contain by nature sharp mixing elements, which potentially could damage sensitive particles. It is also known that the mixing quality is strongly dependent on the flow patterns in the mixer, the relative densities and particle size distributions of the powders to be mixed.